...
First, within Settings, you will need to have the 'Bandwidth Monitoring' module configured for the device type or device type group you'd like to monitor. Go to the 'Device Types' section in 'setup & admin' to Settings to configure the module.
When view the device's page within the Device Manager, you will see that there is now a section called 'Bandwidth Monitoring'. If you do not see this section, ensure that the device's 'type' is correct, and that you have configured the module properly.
...